Nowadays, a technique for making up a thin film transistor (TFT) with a semiconductor thin film (thickness of approximately from several nanometers to several hundred nanometers) formed over a substrate having an insulating surface attracts attention. A thin film transistor is widely applied to an electronic device such as an IC and an electro-optic device. In particular, a thin film transistor has been rushed to be developed as a switching element of a display device.
The study of a thin film transistor using an organic semiconductor (hereinafter, referred to as “organic TFT”) among TFTs is proceeding. An organic TFT has good flexibility since an organic material is used. In addition, an organic TFT can be formed at lower temperature compared with a TFT using an inorganic semiconductor; therefore, a resin material such as plastic can be used for a substrate. As a result, a device which is lightweight and flexible can be obtained. An organic TFT is not only able to be expected to simplify a process by using a printing method, an ink-jet method, a vapor deposition method, and the like but able to suppress a manufacturing cost since an inexpensive material for a substrate can be used; therefore, it can be estimated that there is an advantage in respect of costs.
An organic TFT has a disadvantage that deterioration of an electric characteristic is caused if an organic TFT is left in an atmospheric air since an organic semiconductor is oxidized or decomposed by being exposed to water or oxygen. Therefore, as in Patent Document 1 (Patent Document 1: Patent Laid-Open No. 2003-324202), an insulating film is formed over a semiconductor layer of an organic TFT to protect the semiconductor layer by the insulating film to reduce the deterioration due to water, light, or oxygen.